The musculoskeletal system is subject to injury caused by traumatic events as well as by a number of diseases, such as osteoarthritis and rheumatoid arthritis.
Repair of connective tissue of the musculoskeletal system is commonly performed using materials such as natural or synthetic tendons and ligaments. Joint repair and replacement is typically accomplished using metal and/or polymeric implants and devices. Such devices are typically fixated into existing bone by means of bone plates, adhesives, screws, and the like.
Total joint replacement is indicated under conditions in which the cartilage surface between the bones forming a joint has degenerated. Often it has degenerated to a point where there is significant pain during locomotion, as well as during translation and rotation of joint components. This degenerative joint disease is commonly treated by a technique known as joint surface replacement arthroplasty, which involves replacement of the original surfaces with artificial weight bearing materials in the form of implants.
Hip replacement generally involves the implantation of a femoral component in the form of a ball mounted on a shaft, together with an acetabular component in the form of a socket into which the ball sits.
Total knee replacement is somewhat more difficult than hip replacement because of the complex loading pattern of the knee. The tibial component of a total knee replacement is fixed in the cancellous bone of the tibia. The femoral component is typically fixed to the cortical bone of the femoral shaft using a suitable cement.
The tibial portion of a knee prosthetic device generally involves the insertion of a broad plateau region covering the tibia, after bone of the subchondral plate is removed. In most designs, a composite material is provided, involving a metal support underlying a polymeric, or fiber-reinforced polymeric tray.
A wide array of materials have been described for use in the manufacture of medical implants. See generally, Chapter 1, in Biomaterials, Medical Devices and Tissue Engineering: An Integrate Approach, Frederick H. Silver, ed., Chapman and Hall, 1994. Such materials generally fall into the categories of metals, polymers, ceramics, and composite materials.
A recent article entitled "New Challenges in Biomaterials", Science, 263:1715-1720 (1994), Peppas et al., provides a useful overview of the current state of the art in biomaterials. The article describes a number of materials currently used for orthopedic applications, including metals (iron, cobalt, and titanium), degradable polymers, selfreinforced compositions of polyglycolic acid, stronger polymers such as polydioxanone, and ceramic materials such as hydroxyapatite and certain glasses.
Elsewhere, for instance at page 1719, the Peppas et al. article refers to the potential usefulness of polymers that can be triggered to undergo a phase change. The article itself does not identify such polymers, but instead postulates that materials that are initially liquid might be administered through a minimally invasive surgical device and then triggered to solidify or gel in the presence of ultraviolet light, visible light, or ionic change in vivo. As an example of this approach the article cites an article of Hill-West, et al., Obstet. Gynecol. 83(1):59-64 (1994).
The Hill-West et al. article, in turn, describes the use of a conformable, resorbable hydrogel barrier for preventing postoperative adhesions in animals. The article describes the formation of the hydrogel barrier in situ by photopolymerizing a solution of a macromolecular prepolymer using UV light. The hydrogel barrier is not described as being useful in weight-beating, orthopedic applications, and in fact, was completely resorbed within 7 days after application.
There are a number of drawbacks associated with the biomaterials and related methods presently employed for orthopedic applications, and in particular joint repair and replacement. One such drawback is that these methods generally involve invasive surgery, i.e., resecting tissue in order to gain access to the injury site. In turn, invasive surgery typically involves up to 7 to 10 days of hospitalization, with the costs associated therewith.
A related drawback of an arthrotomy involves the need to cut through skin, nerves, vessels, muscles, ligaments, tendons, and/or joint capsules. Certain procedures can also require the use of either general or spinal anesthesia. They may also require blood transfusions and signifitcant recovery time accompanied by post-surgical pain and discomfort. Lastly, prolonged physical therapy is typically required to strengthen operative areas and prevent contractures. Such therapy can often last up to six weeks or more.
It would be particularly useful to be able to repair such injuries in a manner that avoided such invasive surgical procedures and the problems associated therewith.